As a lead cartridge of this type, a lead cartridge disclosed in Patent Document 1 (Japanese Patent Application examined Publication No. 6-2438) is conventionally known. According to the document, the lead cartridge includes a tip end opening to be detachably inserted into a lead guide, which is provided in a lead feeding mechanism of the mechanical pencil. In the tip end opening, a chuck opening/closing is mounted. The chuck opening/closing mechanism is provided with a lead port, which is opened when the tip end opening is inserted to and engaged with the lead guide and closed when the tip end opening is not engaged.
The chuck opening/closing mechanism includes a pair of elastic opening/closing sections and outwardly engagement sections, which are formed integrally with the elastic opening/closing sections on the tip end side thereof to protrude outwardly in a diameter direction to opposite side to each other and each of which has a tip formed in a round shape. The elastic opening/closing sections form the lead port at the axis center on the tip end side and are elastically deformed to opposite directions to close the lead port by an inward elastic force. When the outwardly engagement sections are press-inserted to and engaged with the lead guide, the outwardly engagement sections move inwardly in a diameter direction against the inward elastic force to open the lead port. When the outwardly engagement sections are pulled out from the lead guide so as to be released from the press-inserted engagement, the outwardly engagement sections move outwardly in the diameter direction by an elastic restoring force to close the lead port.